1. Field of the Invention
The present invention relates to a control apparatus that controls an outboard motor including, as a drive source to rotate a propeller, an engine that discharges exhaust gas in water, and a marine vessel running support system and a marine vessel that are provided with such a control apparatus.
2. Description of Related Art
An outboard motor is one type of propulsion systems for marine vessels, which provides a propulsive force to a marine vessel. In the outboard motor, a motor that generates a drive force to rotate the propeller is provided outboard. An engine type outboard motor is provided with a clutch as a transmission to transmit a drive force generated by an engine to the propeller along with the engine operating as a motor. The shift positions of the clutch are a forward position, a reverse (backward) position and a neutral position. The forward position is a shift position in which a rotation force of the engine is transmitted such that the propeller performs forward rotation (that is, rotation in the direction along which a propulsive force in the forward direction is generated). The reverse position is a shift position in which a rotation force of the engine is transmitted such that the propeller performs reverse rotation (that is, rotation in the direction along which a propulsive force in the reverse direction is generated). The neutral position is a shift position in which no rotation force of the engine is transmitted to the propeller.
A marine vessel is provided with an operation lever used for steering. When the operation lever is in the neutral position, that is, when the operation lever is not operated, the engine idles. Also, the shift position of the clutch is at the neutral position. If the operation lever is moved in the forward direction, the shift position of the clutch is moved into the forward position, and the engine is driven at a target rotational speed corresponding to the movement amount of the operation lever. Therefore, the propeller is rotated (rotated forward) in a direction along which water is pushed out rearward, and a propulsive force which allows a marine vessel to move in a forward direction is generated. On the other hand, if the operation lever is moved in the reverse direction, the shift position of the clutch is moved into the reverse position, and the engine is driven at a target rotational speed corresponding to the movement amount of the operation lever. Therefore, the propeller is rotated (rotated reverse) in the direction opposite to the forward direction, and a propulsive force which allows a marine vessel to move in a reverse direction is generated.
In the engine type outboard motor, exhaust gas generated by the engine is discharged not only in the air but also in the water. Hereinafter, discharge of exhaust gas in the air is called “in-air exhaust,” and discharge of exhaust gas in the water is called “in-water exhaust.”
Regarding in-water exhaust of an engine type outboard motor disclosed in United States Patent Application Publication No. US2004/0203299A1, exhaust gas is discharged through an in-water exhaust port provided in the boss of the propeller.
In an engine type outboard motor disclosed in U.S. Pat. No. 5,529,520, exhaust gas is discharged not only through an in-water exhaust port provided in a boss but also through another in-water exhaust port provided at a position opposed to the boss of the propeller in the casing of the engine.
FIG. 1A is a conceptual view describing in-water exhaust, which shows a state in which a marine vessel moves in a forward direction. FIG. 1B is a conceptual view describing in-water exhaust, which shows a state in which a marine vessel moves in a reverse direction. In a state in which an operator moves the operation lever in the forward direction and a marine vessel moves forward, as shown in FIG. 1A, the propeller rotates in the direction along which water is pushed out rearward. Therefore, bubbles of exhaust gas discharged into the water move away rearwardly. However, if an operator moves the operation lever in the reverse direction, and a marine vessel begins to move in the reverse direction, the propeller rotates in water containing bubbles caused by the exhaust gas (see FIG. 1B). At this time, “bubble entrainment” occurs, by which bubbles are entrained or dragged in the propeller. Therefore, since the amount of water that is pushed out by the propeller is substantially reduced, the propulsion efficiency is reduced. That is, it becomes impossible to obtain a propulsive force corresponding to the rotational speed of the propeller. Furthermore, the higher that the rotational speed of the propeller becomes, the greater that the exhaust amount of the engine is increased. Bubble entrainment is accordingly substantially increased. Therefore, the degree of reduction in the propulsive force resulting from bubble entrainment is increased in accordance with an increase in the rotational speed of the propeller.
In addition, since the accumulation of bubbles decreases as the reverse speed of a marine vessel increases to a certain degree, the amount of bubble entrainment decreases. In other words, as the reverse speed of a marine vessel decreases, the generation of bubble entrainment increases. On the other hand, bubble entrainment occurs not only when a marine vessel moves in the reverse direction but also when the running speed is decelerated by moving the operation lever in the reverse direction at a low forward speed range such as about +2 km/h.
That is, when the operation lever is moved in the forward direction, and the forward speed of a marine vessel exceeds a predetermined speed (for example, 2 km/h), the propulsion efficiency is not reduced due to bubble entrainment. On the other hand, when the operation lever is moved in the reverse direction, and the forward speed of a marine vessel is not more than the predetermined speed, or when a marine vessel moves in reverse, the propulsion efficiency of the propeller is reduced due to bubble entrainment. For this reason, when moving the operation lever in the forward direction and when moving the operation lever in the reverse direction, the amount of movement of the operation lever required to obtain the same propulsive force differs. Therefore, there is a concern that an operator who operates the operation lever may feel a sense of incongruity.